Distance measuring system

ABSTRACT

A device for measuring a moving distance of two relatively moving objects includes a first diffraction grating provided on one of the two objects and disposed along the relatively moving direction of the two objects, and a measuring portion provided on the other object. The measuring portion includes a second diffraction grating, a light source and a photodetecting system, wherein the light source provides lights which are projected upon two points on the second diffraction grating so that they emanate from the two points in the form of diffraction lights having different diffraction orders. The diffraction lights are directed to the same point on the first diffraction grating and are diffracted again by the first diffraction grating so that they are emitted in the same direction, and the photodetecting system is operable to detect a change in the light intensity caused due to the interference of the two lights emanating from the first diffraction grating. The device further includes a detecting system for detecting the relative moving distance of the two objects on the basis of the detection by the photodetecting.

This application is a continuation of application Ser. No. 07/462,291filed Dec. 29, 1989 which is a continuation of application Ser. No.07/190,247 filed May 4, 1988, both now abandoned.

FIELD OF THE INVENTION AND RELATED ART

This invention relates to a distance measuring system and, moreparticularly, to a grating interference type (interferometric) distancemeasuring system.

As disclosed in Japanese Laid-Open Patent Applications, Laid-Open Nos.Sho58-191906 and Sho58-191907, conventional grating interference typedistance measuring devices are arranged so that a coherent light from alight source is directed by way of a mirror or otherwise to adiffraction grating which functions as a reference measure, and positiveN-th order diffraction light and negative N-th order diffraction lightemanating from this diffraction grating are reflected by use of cornercubes backwardly in parallel to their oncoming paths, wherein thereflected diffraction lights are incident again upon the diffractiongrating whereat the two positive and negative N-th order diffractionlights are diffracted in the same direction to cause mutual interferenceand wherein the intensity of such interference light is detected by useof a photosensor.

In the described arrangement, the corner cube is an optical member whichis not easy to manufacture and, therefore, is expensive. Also, theadjustment of optical members such as mirrors is complicated. Therefore,with the conventional arrangement, it is difficult to make the structuresimple and compact and to reduce the manufacturing cost

Further, with such an interferometer, the measuring stroke can be maderelatively large such as of an order of not less than 100 mm. On theother hand, basically, the distance measurement is made by using, as aunit length, a particular pitch which is determined by opticalconditions such as the wavelength of measuring light, the order ofdiffraction light and the state of polarization. Accordingly, theresolution is low and, therefore, the precision is low where a minutedistance of an order not greater than submicrons, for example, is to bemeasured.

SUMMARY OF THE INVENTION

It is accordingly a primary object of the present invention to provide agrating interference type distance measuring device which is simple instructure and which allows easy assembling adjustment.

It is a second object of the present invention to provide a gratinginterference type distance measuring device which allows reduction insize of the device, suppression of noise and improvements in themeasuring precision.

It is a third object of the present invention to provide a gratinginterference type distance measuring device which is stable againstvariation in the wavelength of a light source without use of any cornercube.

It is a fourth object of the present invention to provide a gratinginterference type distance measuring device which has a large stroke andvery high measuring precision.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the structure of a diffractiongrating interferometric distance measuring device according to anembodiment of the present invention, wherein no corner cube is used.

FIG. 2 is a waveform view showing outputs of photodetectors used in theFIG. 1 embodiment.

FIG. 3 is a waveform view of those signals as obtainable by processingthe outputs illustrated in FIG. 2.

FIG. 4 is a schematic view explicating the state of diffraction lightwhere the output wavelength of a light source changes, in the case ofthe distance measuring device of the FIG. 1 embodiment.

FIG. 5 is a waveform view showing outputs of photodetectors of the FIG.1 embodiment where the wavelength of light from the light source isshifted.

FIG. 6 is a schematic view of a grating diffraction type distancemeasuring device according to another embodiment of the presentinvention, wherein a major portion of the device is formed into anintegrated circuit device.

FIG. 7 is an enlarged view showing a major portion of a distancemeasuring device according to a modified form of the FIG. 6 embodiment.

FIG. 8 is a waveform view showing outputs of photodetectors used in theFIG. 7 device.

FIG. 9 is a schematic view showing a distance measuring device accordingto a further modified formed of the FIG. 6 embodiment.

FIG. 10 is a perspective view schematically showing a gratinginterference type distance measuring device according to a yet anotherembodiment of the present invention, wherein a Wollaston prism is used.

FIG. 11 is a schematic view explicating the function of the Wollastonprism of the FIG. 10 device.

FIG. 12 is a schematic view of a grating interference type distancemeasuring device according to a still further embodiment of the presentinvention, wherein light passes through a distance measuring referencegrating by four times so as to improve the resolution of the system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an example of a diffraction grating interferometricdistance measuring device which is constructed without use of any cornercube member as the pulse signal producing means. In FIG. 1, a relativelymovable diffraction grating GS is fixedly provided on one of two objectswhich are movable relative to each other, and a distance measuring headportion MH is fixedly provided on the other of the two objects.

A laser beam emitted from a light source LD (for example, asemiconductor laser) of the distance measuring head portion MH istransformed into a plane wave by means of a collimator lens CL and thenis divided into two by means of a half mirror MH20. The split two lightbeams L01 and L02 are reflected by mirrors MR1 and MR2, respectively,and are incident upon quarter waveplates QW1 and QW2. Thereafter, theyare diffracted by stationary gratings GF1 and GF2, respectively.Positive and negative N-th order diffraction lights LN1 and LN2 areprojected upon the relatively moving grating GS whereat they arereflectively diffracted again, such that they go back in the samedirection and are combined with each other. The thus obtained light isseparated by half mirrors HM21-HM23 and, after being converted intoelectric signals by means of the combination of polarization platesPP1-PP4 and sensors (photodetectors) PD1-PD4, they are extracted. Thequarter waveplates QW1 and QW2 disposed on the paths of the light beamsL01 and L02 are preparatorily set so that their fast axes are inclinedby respective angles +45 degrees and -45 degrees with respect to alinearly polarized light component of the laser beam. Further, theangular positions of the polarizing plates PP1-PP4 are so set that theirorientations of polarization become equal to 0 degree, 45 degrees, 90degrees and 135 degrees, respectively.

With this arrangement, the quantity of each light impinging uponcorresponding one of the sensors PD1-PD4 changes, as shown in FIG. 2,with the movement of the relatively movable grating GS, and thesechanges are detected as light quantity detecting outputs. Namely, fromthe sensors PD1-PD4, output signals whose phases are shiftedsuccessively by a phase difference of 90 degrees.

Then, for each two signals whose phases are relatively shifted by 180degrees, a difference signal is formed. Namely, two difference signalsare formed. These two difference signals have a phase difference of 90degrees and are shown at R and S in upper two parts (a) and (b) of FIG.3.

By use of electric circuits (not shown) and on the basis of apredetermined level, these two signals R and S are binarized(binary-coded) such as depicted in parts (c) and (d) of FIG. 3; and fourpulses per one cycle are produced at the timings of rise and fall of thebinarized signals, as shown in a part (e) of FIG. 3. By counting thenumber of pulses, it is possible to measure the amount of relativemovement between the measuring head MH and the diffraction grating GS.In this case, for the relative movement of an amount corresponding toone pitch of the diffraction grating GS, the interference lightintensity changes through four cycles and, thus, sixteen (16) pulses areproduced. Also, at the time of pulse counting, the direction of saidrelative movement is detected and, in accordance with the result ofdetection, whether or not the counted number should be added orsubtracted is determined. The moving direction can be discriminated fromsuch level of each signal, shown at the parts (c) and (d) of FIG. 3, ascreated at the timing of generation of each pulse shown at a part (e) ofFIG. 3. If, for example, the level of the signal shown in the part (d)as created at the timing of the fall of the signal shown in the part (c)is "high" in an occasion where the movement is in the positivedirection, such level becomes "low" in an occasion where the movement isin the negative or reverse direction.

The signals R and S shown in the parts (a) and (b) of FIG. 3 may beadded and subtracted so as to produce signals "R+S" and "R-S" havingphase differences of 45 degrees with respect to the signals R and S,respectively; these signals may be binarized in a similar manner so thatpulses are produced at the timings of rise and fall. By doing so, it ispossible to obtain thirty-two (32) pulses for the movement of an amountcorresponding to one pitch of the diffraction grating GS.

FIG. 4 shows the state of diffraction lights where, in the distancemeasuring system of the FIG. 1 example, the output wavelength of thelight source LD shifts.

In FIG. 4, the paths of lights where the adjustment is substantiallyperfect are depicted by solid lines, whereas the paths of lights wherethe wavelength is shifted are depicted by dash-and-dot lines. Thus, thelight fluxes as denoted at L11 and L12 in this Figure depict,respectively, the diffraction lights caused when the wavelength has beenshifted. The outputs of the sensors PD1-PD4 when there occurs such awavelength shift are such as depicted in FIG. 5. Independently of theamount of movement of the relatively movable grating GS, a so-calledbias component is included in each of the outputs of the sensors. Thereason for this is that a light flux area (in which no interferencefringe is formed) other than the interference area as depicted byhatching in FIG. 4 increases and that the extent of the light flux areain which no interference fringe is formed changes with the amount ofshift in the wavelength. Accordingly, there occur those changes as beingdepicted in the signal waveforms of the outputs of the photodetectorsPD1-PD4 shown in FIG. 5. However, in an occasion where the processing isgoing to be made on the basis of four detection signals havingsuccessive phase differences of 90 degrees, the division with respect tothe period of signals can be made with good precision even if thewavelength shifts.

Namely, since similar variation components are included in these foursignals, it is possible to cancel such variation components by detectingthe difference between those two signals as having a phase difference of180 degrees. As a result, the DC levels V_(R) and V_(S) of thesedifference signals R and S are not affected by the variation in thewavelength and, therefore, become constant. Accordingly, high precisionis assured when these signals are used for the processing.

If only two sensors are used and if it is desired to obtain pulses ofthe same pitch as in the case using four sensors, by electricallyprocessing two kinds of signals having phases of 0 degree and 90degrees, then the precision of electric division of the signals will bedeteriorated as a result of any shift of the wavelength.

Where light is projected upon a grating with an arrangement such asdisclosed in Japanese Laid-Open Patent Applications, Laid-Open Nos.Sho58-191906 and Sho58-191907, the direction of diffraction thereof(namely the angle) changes with the change in the wavelength of thelight. To meet such characteristics, corner cubes are used. The cornercube is a prism formed to define an angle of 90 degrees between multiplesurfaces so that the reflected light goes back in the same direction asthe incident light. The corner cube requires high machining accuracy sothat it is expensive. Further, the size is large.

In the device of FIG. 1, diffraction grating means (stationary gratingsGF1 and GF2) are provided also on the distance measuring head portion MHside, in addition to the movable grating GS, so that the positive andnegative N-th order diffraction lights from the stationary gratings arediffracted again by the movable grating, the last diffracted lightsadvancing along the same path to the sensor. Accordingly, without use ofany corner cube described above, interference light whose brightnesschanges with the movement of the movable grating is obtainable when thewavelength changes. In other words, the described gratinginterferometric distance meter has good stability to the change in thewavelength without use of any corner cube. Therefore, the cost and sizeof the device can be reduced. Further, the described structurefacilitates integration of the components into a compact device, as willbe described later.

Where a grating interferometric distance measuring device is formed by alight source, a half mirror, corner cubes, polarizing plates, detectorsand so on which are three-dimensionally combined, there is a possibilityof deterioration of the distance measuring accuracy due to inclusion ofan error into an interference signal, as a result of mechanicalvariation between optical components, temperature change or irregularflow of an air. Also, separate provision of the light source, thedetecting system and so on leads to the bulkiness in volume occupying aspace. It is not difficult to make the structure compact. Moreover,because of the spatial distance from the detecting system to theprocessing circuit, noise is easily mixed into the signal, causingdeterioration of the measuring precision.

FIG. 6 shows an example wherein major components of a diffractiongrating distance measuring device are formed into an "integratedcircuit". In this example, a portion which corresponds to an opticalsystem of the distance measuring head portion MH of the distance meterof the FIG. 1 embodiment and a signal processing electric systemeffective to produce pulses in accordance with the brightness/darknessof the interference light, are formed on a base plate of GaAs.

As shown in this Figure, a dielectric waveguide layer WG is formed onthe GaAs base member SB, and the light wave is propagated along a presetoptical path.

Light source LD can be formed on the GaAs base plate SB by use ofmolecular beam epitaxy method, for example. A lens and beam splitterportion LS formed in the waveguide layer WG is effective to transform adiverging light from the light source LD into a parallel light and,then, divide the same along two directions. Grating couplers GC1 and GC2each effective to emit, at a certain angle, the light wave propagatedthrough the thin film waveguide WG, outwardly to the outside space.

Reference diffraction grating GS corresponds to the movable grating GSof the distance meter of the FIG. 1 embodiment, and is effective todiffract the light waves from the grating couplers GC1 and GC2 towardthe same direction. Photodetector PD is provided to detect theinterference light intensity of the diffraction light from the referencediffraction grating GS.

Next, the operation will be described.

The light wave from the light source LD is propagated through thewaveguide WG and, by means of the lens and beam splitter portion LS, itis transformed into two parallel lights L01 and L02 which are propagatedthrough the waveguide WF in different directions. Each of the lights L01and L02 is reflected within the waveguide WG by corresponding one ofmirrors MR1 and MR2 so that it advances in parallel to the lengthwisedirection of the reference grating GS. The reflected lights from themirrors MR1 and MR2 are incident on the grating couplers GC1 and GC2.The grating couplers GC1 and GC2 each functions to emit the light wave,having been propagated through the waveguide WG, from the surface of thebase plate to the outside at a preset angle and through the waveguidesurface. This angle is related to the pitch of the reference grating GSand the wavelength of the light. If a reference grating having a pitchp=1.6 micron is used, and where the wavelength λ=0.83 micron, then theangle of emission is 58.8 degrees.

The two light waves from the grating couplers GC1 and GC2 areperpendicularly diffracted by the reference diffraction grating GS andare incident on the photodetector PD. The photodetector PD operates tophotoelectrically convert the interference intensity of the twodiffraction lights.

Next, the principle of operation as a distance meter will be explained.

The light waves emitted to the outside space by means of the gratingcouplers GC1 and GC2 are diffracted by the reference grating GS, asdescribed. The intensity distribution of the diffraction light producedat that time can be expressed by the following equation:

    I=I.sub.0 +I.sub.1 cos[2π·X/{p/(m-n)}]

wherein

X: the amount of relative change between the base plate and thereference grating;

p: the pitch of the reference diffraction grating;

m: the order of diffraction, by the reference diffraction grating of thelight from the grating coupler GC1;

n: the order of diffraction, by the reference diffraction grating of thelight from the grating coupler GC2;

I₀ : the DC level; and

I₁ : the signal amplitude.

Assuming now that m=+1, n=-1 and p=1.6 micron, then the intensitydistribution I can be determined by:

    I=I.sub.0 +I.sub.1 cos[2(X/0.8)]

It is seen therefrom that, each time the reference grating GS movesthrough 0.1 micron pitch, a sine wave signal of one cycle is produced.The detector PD is operable to count the cycles of such sine wavesignals, so that the amount of movement of the reference grating GS canbe measured.

The grating interference type distance measuring device of the presentembodiment has a light source, optical members and a detection systemprocessing circuit which are integral on the same base plate.Accordingly, the size can be reduced and the noise can be suppressedand, additionally, higher precision is attainable.

Description will now be made of the means for detecting the movingdirection of the reference grating GS.

In order to detect the moving direction, it is necessary to obtain twosignals whose phases are relatively shifted by an amount correspondingto one-fourth (1/4) of the cycle.

A specific example is illustrated in FIG. 7, wherein the referencegrating GS is formed by two grating arrays GL1 and GL2 whose phases arerelatively shifted with respect to the moving direction of the referencegrating GS by an amount corresponding to 1/4×(m-n) pitch. Additonally,two photoelectric detectors PD1 and PD2 are formed on the base plate SB,correspondingly to the two grid arrays.

The diffraction lights from the grid arrays GL1 and GL2, respectively,are received by the different sensors PD1 and PD2 which are spatiallyseparated. By this, signals whose phases are relatively shifted byone-fourth (1/4) of the cycle, such as shown in FIG. 8, are obtainable.

FIG. 9 shows an example wherein a grid interference type distancemeasuring device is provided as an optical heterodyne measurementdevice.

In this example, a frequency shifter FS which comprises a surfaceacoustic wave device, for example, is disposed at the middle of theoptical path so that a light wave whose frequency is shifted, withrespect to the frequency f₀ of the output light from the light sourceLD, by an amount Δf corresponding to the oscillation frequency of anoscillator OSC. Light waves of the frequency f₀ and the frequency f₀ +Δfare projected upon grating couplers GC1 and GC2 and, by way of thesecouplers, they are projected upon a reference grating GS having a singlegrating array. The light diffracted by the reference grating GS isreceived by a photodetector PD.

The signal which can be directly obtained by the photodetector PD can beexpressed as follows:

    I=I.sub.0 +I.sub.1 cos[2π·X/{p/(m-n)}]

It is seen therefrom that, by detecting a phase difference with respectto an output signal from the oscillator OSC by use of a phase detectingcircuit PSD, the amount of movement of the reference grating GS and themoving direction thereof can be detected as in the case of the foregoingembodiment.

One of the features of the device of the present embodiment lies in thatthere is no necessity of use of a special grating (see FIG. 7 forexample) for the discrimination of the moving direction. Additionally,in a short time, the averaging with respect to time is attainable.Therefore, the amount of movement can be detected very precisely.

In the integrated circuit type distance measuring devices of the FIGS. 6and 9 examples, a GaAs member is used as the base plate SB. However, thebase member may be made of Si. In such case, the light source LD may beprovided outwardly.

As described hereinbefore, by integrally providing an optical system(excluding a reference grating) and a signal processing electric systemupon a single base member, in a grid interference type distancemeasuring device, the necessity of assembling adjustment is eliminatedand the device can be made stable against disturbance. Further, the sizeand weight of the device can be reduced, while assuring high-precisionmeasurement.

Usually, in a grid interference type distance measuring device, anoptical system includes mirrors or corner cubes. Particularly, mirrorsor otherwise is used in an optical system for projecting light upon agrating. However, this leads to a difficulty in the assemblingadjustment and a difficulty in making the device compact.

FIG. 10 shows an example wherein a double refraction prism such as aWollaston prism is used so as to project a light upon a relativelymoving grating, such that an optical system for directing the light tothe grating is made simple in structure.

In FIG. 10, a light emitted from a light source LD such as asemiconductor laser or otherwise is transformed into a plane wave bymeans of a collimator lens CL, and the plane wave thus formed isperpendicularly incident upon a Wollaston prism WP. The Wollaston prismis formed by cementing two double refraction (birefringence) materialmembers (e.g. calcite members) shaped like a prism. The light incidentupon the Wollaston prism is divided into two polarized light componentsperpendicular to each other, and both of the light components can beextracted. FIG. 11 shows this. The light to be projected upon theWollaston prism WP may be a linearly polarized light having a directionof polarization inclined by 45 degrees with respect to a P-polarizedlight L0p and an S-polarized light L0s or, alternatively, a circularlypolarized light which may be provided by interposing a quarter waveplatebetween the collimator lens CL and the Wollaston prism WP.

In FIG. 11, the lights emanating from the Wollaston prism WP are suchthat their P-polarized light components and S-polarized light componentshave the same angle of incidence with respect to the grating GS, but theangles of incidence of these polarized light components have oppositesigns. When these lights pass through a quarter waveplate QW, theP-polarized light and the S-polarized light are converted intocircularly polarized light having opposite rotational directions. Thesecircularly polarized lights can spatially interfere with each other. Theinterfering light is divided by a beam splitter BS into two beams whichare directed to two photodetectors PD1 and PD2, respectively, havingpolarizing plates PP1 and PP2 disposed in front of them, respectively.By doing so, signal outputs such as illustrated in FIG. 3 are obtainedand, by effecting the electric processing having been described withreference to the device of FIG. 1, signals of the grating interferencetype distance meter are obtained. The polarizing plates PP1 and PP2 havetheir polarization axes shifted by 45 degrees relative to each other.

In the device of FIG. 10, a Rochon prism, a Glan-Thompson prism orotherwise may be used as the double refraction prism. However, wherethese prisms are used, the relation between the prism used and the lightincident upon differs from the relation (perpendicular incidence) asestablished when a Wollaston prism is used.

FIG. 12 shows an example of a grating interference type distancemeasuring device wherein corner cubes are used to bend or deflectoptical paths so that each diffraction light goes and returns twice,whereby the number of light division by a distance measuring referencegrating GS is increased to eight (8) with a result of increasedresolution.

In a distance meter of such structure as disclosed in the aforementionedJapanese Laid-Open Patent Applications, Laid-Open Nos. Sho58-191906 andSho58-191907, for example, the quantity of light on a photosensorchanges at intervals corresponding to one-fourth (1/4) of the pitch of aused grating, such as depicted by signals R and S shown in the parts (a)and (b) of FIG. 3. In the grating interference type distance meterdisclosed in these Japanese Patent Applications, the period of such alight quantity detection signal (R or S) is electrically divided toincrease the number of pulse signals per one pitch of the grating tothereby improve the resolution. However, where the division is made byelectrical processing, there is a possibility that the pulse spacingchanges with the change in the amplitude of a signal or in the DC level.If this occurs, the precision is degraded.

In the present embodiment, as compared therewith, the optical system ofthe measuring device is arranged so that the number of times of lightdiffraction at the distance measuring reference grating GS is increasedwith a result that the quantity of light upon a photosensor changes, bymany times (e.g. eight times), during a time period during which thereference grating GS moves by an amount corresponding to one pitchthereof. With this arrangement, the quantity of light upon thephotosensor changes at very short intervals such as, for example,one-eighth of the pitch of the reference grating. Thus, with the opticalarrangement itself, the number of divisions with regard to the grating(grating pitch) is increased.

In FIG. 12, the light emanating from a light source LD, which comprisesa semiconductor laser, for example, of the grating interference typedistance measuring optical system is transformed into a plane wave lightL0 by means of a collimator lens CL and then impinges on a point P1 onthe distance measuring reference grating GS which is in a relativelymovable relation with the distance measuring optical system. The lightincident on the reference grating GS is diffracted thereby. Positive andnegative N-th order diffraction lights L11 and L12 caused thereby enterinto corner cubes CC1 and CC2, respectively, and are reflected thereby,each reflected light travelling in a direction parallel to its oncomingpath and reversely. The reflected lights from the corner cubes CC1 andCC2 are incident again on the reference grating GS at points P2 and P3,respectively, and are diffracted again by the grating GS. Thesediffracted lights denoted at L21 and L22 pass through phase plates FP1and FP2, respectively, so that the state of polarization of each lightis changed. After reflected by corner cubes CC3 and CC4, the lights L21and L22 come back to the grating GS at points P4 and P5 and arediffracted again by the grating GS. These diffraction lights as denotedat L31 and L32 are reflected again by the corner cubes CC1 and CC2,respectively, and they come back to the grating GS and are incident uponthe same point P6 whereat they are diffracted again (the fourthdiffraction). The fourth-diffracted lights denoted at L41 and L42interfere with each other. The interfering light goes to a beam splitterHM by way of a mirror MR, and they are separated into two which aredirected by way of polarizing plates PP1 and PP2 to sensors PD1 and PD2,respectively.

The phase plates FP1 and FP2 may comprise quarter waveplates, forexample, and are set so that their fast axes are inclined at angles +45degrees and -45 degrees with respect to the linearly polarizedcomponents of the laser beams L21 and L22, respectively. Also, thepolarizing plates PP1 and PP2 may be set so as to have angles 0 degreeand 45 degrees, respectively. With the above-described arrangement,those signals as having intensities varying with a phase difference of90 degrees are obtainable at the two sensors PD1 and PD2. Further, wherethe pitch of the distance measuring reference grating is 2.4 microns andwhen the order of diffraction at each of various points and times is"±1-st", there are produced at the sensors PD1 and PD2 those signalshaving 0.3 micron spacing which is one-eighth (5/8) of the pitch of thegrating. By dividing the thus determined pulse interval in accordancewith the electrical division method described with reference to FIG. 1,for example, pulse signals of a number twice larger than the pulsenumber just described, that is thirty-two pulses per one pitch withinterval of 0.075 microns, are obtainable.

While the invention has been described with reference to the structuresdisclosed herein, it is not confined to the details set forth and thisapplication is intended to cover such modifications or changes as maycome within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is:
 1. A device for measuring a relatively movingdistance of two relatively moving objects, said device comprising:alight source provided on one of the two objects; an opticalintegrated-circuit provided on the one object at a position opposed tothe other object, said optical integrated-circuit including a baseplate, a waveguide layer formed on said base plate, dividing meansprovided in said waveguide layer, for dividing a light from said lightsource into two lights, and emitting means effective to emit each of thetwo lights defined by said dividing means, outwardly of said waveguidelayer and in a predetermined direction; a diffraction grating providedon the other object at a position opposed to said opticalintegrated-circuit, for diffracting each of the lights emitted by saidemitting means; photodetecting means for detecting a change in the lightintensity caused by the interference of the two diffraction lightsemitted from said diffraction grating; and detecting means for detectingthe relatively moving distance of the two objects on the basis of thedetection by said photodetecting means.
 2. A device according to claim1, wherein said light source is provided on said base plate.
 3. A deviceaccording to claim 1, wherein said diffraction grating includes twograting element arrays disposed so that they are relatively shifted inthe direction of relative movement of the two objects by an amountcorresponding to one-fourth of the cycle of the change in the lightintensity due to the interference of the diffraction lights and whereinsaid photodetecting means is operable to detect the changes in the lightintensity of the diffraction lights from said grating element arrays,independently of each other.
 4. A device according to claim 1, furthercomprising a frequency shifter for shifting the frequency of one of thetwo lights defined by said dividing means and means for detecting thedirection of relative movement of the two objects by detecting a phasedifference of an output of said photodetecting means with respect to thefrequency as shifted by said frequency shifter.
 5. A device formeasuring a relatively moving distance of two relatively moving objects,said device comprising:a light source provided on one of the twoobjects; a diffraction grating provided on the other object, forreceiving and diffracting a light emitted from said light source;reflecting means for reflecting two diffraction light having differentdiffraction orders and emitted from said diffraction grating indifferent direction so that each of the two diffraction lights isfurther projected, by n times, upon said diffraction grating and isdiffracted thereby, wherein n is an odd number not less than 3 andwherein, after the n times diffraction, the two diffraction lightsemanate from said diffraction grating in the same direction;photodetecting means for detecting a change in the light intensitycaused by the interference of the two diffraction lights emitted in thesame direction from said diffraction grating; and detecting means fordetecting the relative moving distance of the two objects on the basisof the detection by said photodetecting means, wherein said reflectingmeans includes first and second corner cube prisms effective to reflect,respectively, the two lights from said light source and diffracted bysaid diffraction grating and third and fourth corner cube prismseffective to reflect, respectively, the lights having been reflected byrespective corner cube prisms and diffracted by said diffractiongrating, and wherein the lights reflected respectively by said third andfourth corner cube prisms are diffracted by said diffraction gratingand, then, are reflected by said first and second corner cube prisms,respectively, so that they are incident upon the same location on saiddiffraction prism and diffracted thereby, such that they emanate fromsaid diffraction grating in the same direction.
 6. A device formeasuring relative movement of two relatively movable objects, saiddevice comprising:a light source means provided on one of the objects,for emitting light; a diffraction grating provided on the other objectand diffracting light from said light source means; first detectingmeans for detecting a change in intensity of light diffracted by saiddiffraction grating, due to optical interference; second detecting meansfor detecting relative movement of the first and second objects, on thebasis of detection by said first detecting means; and an opticalintegrated element provided on the one object, for forming a part of apath for the light travelling from said light source means to said firstdetecting means.
 7. A device according to claim 6, wherein said opticalintegrated element forms a path from said light source means to saiddiffraction grating.
 8. A device according to claim 7, wherein saidoptical integrated element includes dividing means for dividing lightfrom said light source means into two beams, and wherein said opticalintegrated element directs the two beams to said diffraction grating. 9.An encoder, comprising:a diffraction grating; and a reading headrelatively displaceable to said diffraction grating, wherein saidreading head includes; light inputting means for inputting coherentlight to said diffraction grating for first time impingement thereon;first reflecting means for receiving light diffracted by saiddiffraction grating and for emitting the received light for second timeimpingement upon said diffraction grating; and second reflecting meansfor receiving light diffracting by said diffraction grating in responseto the second time impingement and for emitting the received light forthird time impingement upon said diffraction grating, wherein saidsecond reflecting means is in such positional relationship with saidfirst reflecting means that light diffracted by said diffraction gratingin response to the third impingement is inputted through said firstreflecting means again to said diffraction grating for fourth timeimpingement thereon, wherein each of said first and second reflectingmeans has a function for defining a path for light emission which iscodirectional to a path for light reception irrespective of the angle ofincidence of the light received thereby, and wherein said reading headfurther includes photodetecting means for producing an outputcorresponding to a product of interference of light based on thediffraction by said diffraction grating responsive to the fourth timeimpingement with light supplied from said light inputting means.
 10. Anencoder according to claim 9, wherein said photodetecting means receivesthe light diffracted by said diffraction grating in response to thefourth time impingement.
 11. An encoder according to claim 9, whereinsaid first and second reflecting means are in such positionalrelationship that a path of the light resulting from the diffraction bysaid diffraction grating and being directed to said photodetecting meansand a path of light defined by said light inputting means do not overlapwith each other.
 12. An encoder according to claim 9, wherein said lightinputting means includes a semiconductor laser.
 13. An encoder accordingto claim 9, wherein each of said first and second reflecting meanscomprising a corner cube.
 14. A reading head for use in an encoderhaving a diffraction grating, said reading head being relativelydisplaceable to the diffraction grating, said reading headcomprising:light inputting means for inputting coherent light to thediffraction grating for first time impingement thereon; first reflectingmeans for receiving light diffracted by the diffraction grating and foremitting the received light for second time impingement upon thediffraction grating; second reflecting means for receiving lightdiffracted by the diffraction grating in response to the second timeimpingement and for emitting the receiving light for third timeimpingement upon the diffraction grating, wherein said second reflectingmeans is in such positional relationship with said first reflectingmeans that light diffracted by said diffraction grating in response tothe third impingement is inputted through said first reflecting meansagain to the diffraction grating for fourth time impingement thereon,wherein each of said first and second reflecting means has a functionfor defining a path for light emission which is codirectional to a pathfor light reception irrespective of the angle of incidence of the lightreceived thereby; and photodetecting means for producing an outputcorresponding to a product of interference of light based on thediffraction by the diffraction grating responsive to the fourth timeimpingement with light supplied from said light inputting means.
 15. Areading head according to claim 14, wherein said photodetecting meansreceives the light diffracted by the diffraction grating in response tothe fourth time impingement.
 16. A reading head according to claim 14,wherein said first and second reflecting means are in such positionalrelationship that a path of the light resulting from the diffraction bythe diffraction grating and being directed to said photodetecting meansand a path of light defined by said light inputting means do not overlapwith each other.
 17. A reading head according to claim 14, wherein saidlight inputting means includes a semiconductor laser.
 18. A reading headaccording to claim 14, wherein each of said first and second reflectingmeans comprises a corner cube.